The present invention relates to a color image reading apparatus employing, as reading light sources, light emitting diodes in three colors, respectively.
A prior art color image reading apparatus employing, as reading light sources, light emitting diodes in three colors has such a design as shown in FIG. 4. Specifically, the respective anodes of a red, a green and a blue light emitting diodes 31R, 31G, 31B are commonly connected to a 5-volt DC power source for example. The cathode of each light emitting diode 31R, 31G, 31B is connected to one end of a respective resistor 32R, 32G, 32B. The other end of each resistor 32R, 32G, 32B is connected to a respective analog switch 33R, 33G, 33B. The other end of each analog switch 33R, 33G, 33B is commonly grounded.
Each light emitting diode 31R, 31G, 31B is controlled individually by adjusting the on-period of the respective analog switch 33R, 33G, 33B, such that the amount of light emitted from the respective light emitting diode has a predetermined value.
The light emitting diodes 31R, 31G, 31B differ largely from product to product with respect to the correlation between the current and the light output. Therefore, the light output of each diode has to be controlled by adjusting the on-period depending on the light output or by regulating the current depending on the on-period in order to obtain a predetermined amount of light emission. In the color image reading apparatus described above, the on-period is adjusted depending on the light output. Such adjustment is performed before shipment of the color image reading apparatus from the factory.
However, due to product variations, some light emitting diodes may have an extremely low light output. A color image reading apparatus employing such a light emitting diode necessitates a sufficiently long on-period, consequently giving rise to problems in realizing high-speed reading. Further, a difference in on-period between the red, green and blue colors results in a difference in moving distance, during the on-period, of an object being scanned in the secondary scanning direction, whereby the color reading quality may be adversely affected.
Moreover, since the prior art apparatus is incapable of reducing the current for monochrome reading, the life of e.g. the green light emitting diode 31G turned on for monochrome reading may be shortened.
For clarity improvement in monochrome reading, only the green light emitting diode 31G is turned on instead of simultaneously turning the light emitting diodes 31R, 31G, 31B for the three colors. In this case, since the green light emitting diode 31G, which is also used for full-color reading, is driven for monochrome reading with the same large current as used for full-color reading, the life of the green light emitting diode 31G may be shortened.
More specifically, due to differences in focal distance resulting from the different wavelengths of the three colors, the focal distance of the lens, which is used for focusing light reflected from the object surface at the light receiving surface of respective photoelectric elements, is normally adjusted with respect to the green light which has an intermediate wavelength among the red, green and blue light. Therefore, if the red, green and blue light emitting diodes 31R, 31G, 31B are turned on together for monochrome reading, the red light and the blue light are not focused accurately at the light receiving surface due to the focal distance differences between the three colors, thereby causing the image profile to be unclear. This is why the green light emitting diode 31G alone is turned on for monochrome reading.
It is therefore an object of the present invention to provide a color image reading apparatus which is capable of providing high-speed reading regardless of output fluctuations between different light emitting diodes while also prolonging the life of a light emitting diode which is used for monochrome reading.
According to a first aspect of the present invention, there is provided a color image reading apparatus comprising: a first lighting controller which, in full-color reading, successively turns on a red light emitting diode, a green light emitting diode and a blue light emitting diode as reading light sources each for a predetermined period; a first light output controller for setting light output of each light emitting diode, which is turned on by the first lighting controller, to a first predetermined level in full-color reading; a second lighting controller for continuously turning on one of the light emitting diodes in monochrome reading; and a second light output controller for setting, in monochrome reading, light output of said one light emitting diode, which is turned on continuously by the second lighting controller, to a second predetermined level less than the first predetermined level.
One or more light emitting diodes may be provided for each color.
The first predetermined level may be different or equal for each color.
The first lighting controller, the first light output controller, the second lighting controller and the second light output controller may be provided by a CPU (central processing unit) operating on predetermined programs.
The green light emitting diode may be turned on continuously at the time of monochrome reading. However, such is not limitative.
In a preferred embodiment, the second lighting controller turns on the green light emitting diode, and the second light output controller sets the light output of the green light emitting diode to the second predetermined level.
In another preferred embodiment, a single light emitting diode is provided for each color.
According to a second aspect of the present invention, there is provided a color image reading apparatus comprising: red, green and blue light emitting diodes provided, in any number for each other, as reading light sources; a variable current regulating circuit for supplying current to the light emitting diodes; a switch circuit for turning on and off each light emitting diode; a first lighting controller which controls the switch circuit for turning on the light emitting diodes, successively with respect to the colors, for a predetermined period in full-color reading; a first light output controller which, in full-color reading, controls the variable current regulating circuit in synchronism with the first lighting controller according to first reference data stored beforehand in a first storage device for setting the light output of each light emitting diode to a first predetermined level; a second lighting controller which controls the switch circuit for continuously turning on a selected color of the light emitting diodes in monochrome reading; a second light output controller which, in monochrome reading, controls the variable current regulating circuit according to second reference data stored beforehand in a second storage device for setting the light output of the selected light emitting diode, which is continuously turned on by the second lighting controller, to a second predetermined level less than the first predetermined level.
The switch circuit may comprise analog switches, but such is not limitative.
Each of the first and second storage device may comprise an EEPROM (electrically erasable and programmable read only memory) for example. However, a flash memory or a RAM (random access memory) backed up by a rechargeable battery may also be used. The first and second storage devices may be incorporated on a common semiconductor chip or separately on different semiconductor chips.
In a preferred embodiment, the switch circuit comprises an analog switch.
In another preferred embodiment, the first storage device comprises an electrically erasable non-volatile memory.
In another preferred embodiment, the second storage device comprises an electrically erasable non-volatile memory.
In another preferred embodiment, the first and second storage devices are incorporated on a single semiconductor chip which provides an electrically erasable non-volatile memory.